This invention relates to an improved method for recovery of ethylene and more particularly to the addition of a secondary deethanizer to an existing ethylene plant having at least one deethanizer for the improved fractionation of ethane and ethylene.
Ethylene is a building block in the manufacturing of a wide variety of chemical materials and is typically produced industrially by pyrolysis of hydrocarbons in a furnace in the presence of steam. The furnace effluent stream comprising a range of components is typically cleaned up, dried to remove water, compressed and passed to an olefins recovery section to separate the ethylene from other light hydrocarbons, such as ethane, propylene, propane, and the like.
The order of removal of compounds from the furnace effluent can begin with either a demethanizer, deethanizer, or a depropanizer. FIG. 1 illustrates a flow pattern that is generally representative of prior art demethanizer-first fractionation processes. The effluent 10 from the cracking furnaces (not shown) can be cooled and compressed and enters the first fractionation tower, demethanizer 12, which removes methane and lighter compounds. The light compounds exit the overhead of the demethanizer 12 via line 14 and can enter a cold box 16 where they can be separated into a hydrogen rich tail gas stream 18 and a methane-rich gas stream 20. The remainder of the furnace effluent leaves the bottom of the demethanizer via line 22 and is supplied to the deethanizer 24.
The deethanizer 24 separates the demethanizer bottoms effluent into C2s and lighter compounds which exit the overhead of the deethanizer 24 as vapor via line 26 and a heavier portion which exits via line 28 as deethanizer bottoms. The overhead C2s can be cycled through a reactor (not shown) to remove any acetylene and then fed to a C2 splitter 30 to produce ethylene-rich stream 32 and ethane-rich stream 34.
The deethanizer bottoms are fed to the depropanizer 36 where C3s and lighter compounds exit the overhead of the depropanizer via line 38 and can be supplied to a reactor (not shown) to remove methyl acetylene and propadiene. The C3s are then supplied to a C3 splitter 42 which separates propylene 44 and propane 46. The depropanizer bottoms are fed to a debutanizer via line 40. The debutanizer 48 recovers the C4s as a mixed overhead effluent 50. C5 and heavier compounds which exit as debutanizer bottoms via line 52 can be recycled to the cracking furnaces (not shown).
In U.S. Pat. Nos. 5,678,424 and 5,884,504, Nazar discloses improvements to the fractionation of ethylene by adjusting the number of rectification trays. In U.S. Pat. Nos. 5,709,780 and 5,755,933, Ognisty et al. disclose a partitioned distillation column combining a distillation column with an upstream stripper/absorber. All patents and publications referred to herein are hereby incorporated by reference in their entirety.
A major operating consideration in existing deethanizer columns is the strict requirement for low C2 content in deethanizer bottoms. Typically, prior art deethanizers require a C2 concentration in the deethanizer bottoms of less than 200 mol ppm. To achieve such a high recovery rate in the deethanizer with a front-end demethanizer design, the bottoms are often reboiled at high temperatures, resulting in high fouling rates in the reboiler. Currently, to address the high fouling rates in ethylene plants, multiple redundant reboilers can be installed.